1. Technical Field
This disclosure relates generally to hydraulic fracturing and more particularly to systems and methods for spare turbine power generation, which is sometimes referred to as reserve power.
2. Background
With advancements in technology over the past few decades, the ability to reach unconventional sources of hydrocarbons has tremendously increased. Horizontal drilling and hydraulic fracturing are two such ways that new developments in technology have led to hydrocarbon production from previously unreachable shale formations. Hydraulic fracturing (fracturing) operations typically require powering numerous components in order to recover oil and gas resources from the ground. For example, hydraulic fracturing usually includes pumps that inject fracturing fluid down the wellbore, blenders that mix proppant into the fluid, cranes, wireline units, and many other components that all must perform different functions to carry out fracturing operations.
Usually in fracturing systems the fracturing equipment runs on diesel-generated mechanical power or by other internal combustion engines. Such engines may be very powerful, but have certain disadvantages. Diesel is more expensive, is less environmentally friendly, less safe, and heavier to transport than natural gas. For example, heavy diesel engines may require the use of a large amount of heavy equipment, including trailers and trucks, to transport the engines to and from a wellsite. In addition, such engines are not clean, generating large amounts of exhaust and pollutants that may cause environmental hazards, and are extremely loud, among other problems. Onsite refueling, especially during operations, presents increased risks of fuel leaks, fires, and other accidents. The large amounts of diesel fuel needed to power traditional fracturing operations requires constant transportation and delivery by diesel tankers onto the well site, resulting in significant carbon dioxide emissions.
Some systems have tried to eliminate partial reliance on diesel by creating bi-fuel systems. These systems blend natural gas and diesel, but have not been very successful. It is thus desirable that a natural gas powered fracturing system be used in order to improve safety, save costs, and provide benefits to the environment over diesel powered systems. Turbine use is well known as a power source, but is not typically employed for powering fracturing operations.
Though less expensive to operate, safer, and more environmentally friendly, turbine generators come with their own limitations and difficulties as well. As is well known, turbines generally operate more efficiently at higher loads. Many power plants or industrial plants steadily operate turbines at 98% to 99% of their maximum potential to achieve the greatest efficiency and maintain this level of use without significant difficulty. This is due in part to these plants having a steady power demand that either does not fluctuate (i.e., constant power demand), or having sufficient ing if a load will change (e.g., when shutting down or starting up a factory process).
In hydraulic fracturing, by contrast, the electrical load constantly changes and can be unpredictable. This unpredictability is due to the process of pumping fluid down a wellbore, which can cause wellhead pressure to spike several thousand PSI without warning, or can cause pressure to drop several PSI unexpectedly (sometimes called a “break,” as in the formation broke open somewhere). In order to maintain a consistent pump rate, the pump motors are required to “throttle” up or “throttle” down (applying more or less torque from a variable frequency drive), drawing either more or less electrical power from the turbines with little to no notice in many situations.
Concurrently with pressure variations, fluid rate variations can also occur. At any moment, the contracting customer may ask for an extra 5 barrels per minute (bpm) of pump rate or may request an instantly decreased pump rate with little to no warning. These power demand changes can vary from second to second—unlike industrial power demands, which may vary from hour to hour or day to day, allowing for planning and coordination.
Hydraulic horsepower (HHP) can be calculated with the following relationship:
      H    ⁢                  ⁢    H    ⁢                  ⁢    P    =                    (                  Wellhead          ⁢                                          ⁢          Pressure                )            ×              (                  Pump          ⁢                                          ⁢          Rate                )              40.8  HHP also directly correlates with the power demand from the turbines, where:HHP≈Electrical Power DemandTherefore, if both variables (rate and pressure) are constantly changing, maintaining a steady power demand can be difficult. Due to this, it is impossible to design the equipment and hold the turbine output at 98%-99% of full potential because a minute increase in power demand may shut the turbines down and may result in failure of the fracturing job. To prevent such turbine shutdown from happening, fracturing equipment s designed to only require approximately 70% of the maximum output of the turbine generators during normal and expected operating conditions. This allows the fleet to be able to operate against changing fracturing conditions, including increased fluid rate and increased wellhead pressure.
There are also other small loads which contribute to changing power demand. These include turning on or off small electrical motors for hydraulic pumps, chemical pumps, cooling fans, valve actuators, small fluid pumps, etc., or power for metering instrumentation, communication equipment, and other small electronics. Even lighting or heating can contribute to the fluctuating power load.
Therefore it tray be desirable to devise a means by which turbine power generation can be managed at an output usable by fracturing equipment.